This invention relates to R.F. network analyzers, and particularly to very broadband R.F. directional bridges for performing vector measurements and time domain reflectometry.
Characterizing circuits in the frequency domain is a fundamental activity in both the design and testing of electronic circuits. Only by knowing the gain/phase-vs-frequency characteristics and the input-output impedances of each circuit can one assemble a complete device capable of meeting performance objectives. In recent years, significant progress has been made in the development of network analyzers to characterize component and circuit performance. By providing plots of gain or loss, phase shift and reflection coefficients versus frequency, these instruments have given electronic engineers in depth practical insight into circuit and component behavior. In addition, with better computational devices and wider bandwidth measuring instruments, time domain reflectometry is more readily available which further enhances insight into circuit behavior and leads to more precise designs. In turn, these designs have led to tighter system performance requirements, for example, closer packing of communications channels and this in turn has led to a demand for even better measuring instruments. At the heart of such measuring instruments is the directional bridge for separating reflected and transmitted signals from incident signals, e.g., in order to characterize the S-parameters of a device under test (D.U.T.). Such a prior art bridge is shown in FIG. 1. Here the device is typically symmetric relative to the R.F. input, with the resistance of resistors R1 and R2 being equal to each other, and equal in value to that of load resistor R0, i.e., R1=R2=R0. The device is typically implemented with a balun B between reference port (REF) and test port (T), and with a termination resistor R3=R0 for extracting the signal. As a practical matter, however, the circuit of FIG. 1 is highly idealized. If the balun and termination are replaced by equivalent realistic impedances, the difficulties in constructing such a device become readily apparent. A corresponding equivalent circuit is shown in FIG. 2. Here, R1=R2=R0 as before. However, for satisfactory measurements the following requirements must be satisfied: (1) the parasitic shunt impedances ZB and ZC must be very large, i.e., ZB&gt;&gt;R0 and ZC&gt;&gt;Z D.U.T. where Z D.U.T. is the impedance of the device under test; or (2) ZB and ZC must be extremely well balanced, i.e., ZB=ZC exactly; or (3) ZB and ZC must satisfy some relation which is a compromise between (1) and (2) above. For measuring instruments which are restricted to a few octaves or less in bandwidth, these restrictions can usually be met without extreme measures. However, above 1 to 2 GHz, it becomes very difficult to produce a broadband impedance greater than about 1 K ohm, since the impedance decreases rapidly with increasing frequency. Hence, higher frequencies impose very stringent requirements for balancing of the two shunt impedances. As a result, for very wide bandwidth devices, e.g., over the entire range from 45 MHz to 26.5 GHz, it does not appear physically possible with the present state of the art to provide such high impedances or such precise balancing. Nevertheless, wide bandwidth directional bridges are highly desirable for automated test equipment using broadband sweepers to characterize circuit parameters and are particularly important for performing Fourier transforms for accurate time domain reflectometry, a technique which has proven extremely useful in solving intricate design problems. To date, the practical difficulties of extracting the desired differential signal over such a wide bandwidth without introducing unbalanced parasitic impedances has eluded the industry.